Boron nitride (BN) can exist in an amorphous mode or a crystalline mode. Crystalline BN can exist in three forms; hexagonal, cubic, and wurtzite modification. The hexagonal form is similar in structure to graphite, consisting of a simple sheet structure composed of fused borazine rings. This form of BN is colorless when pure and sublimes at about 3000.degree. Centigrade. It is chemically inert except to extremely hot alkali and fluorine.
The second allotrope of BN is the cubic form which is analogous to diamond in its properties, structure (sphalerite), and preparation. If properly synthesized, the cubic form of BN is hard enough to scratch diamond.
The third form of crystalline BN is the wurtzite modification, which is not as hard as cubic but harder than the hexagonal form.
The cubic form is prepared by subjecting the hexagonal form to high pressure (for example, 85,000 atmospheres) and heat (1800.degree. Centigrade). See, for example, Mazurenko, Sin. Almazy. 3, 3 (1979). Alkali metal or alkaline earth metals are known to catalyze this transformation at lower pressures and temperatures. See Meller, Gmelin Handbook der Anorganische Chemie Boron Compounds, 2nd Suppl., 1, 304 (1983).
The wurtzite modification is conveniently made by shock wave-induced transformation of the hexagonal material, as taught by Saito, Proc. Intern. Symp. Factors Densif. Sintering Oxide non-oxide Ceram., Hakone, Japan (1979).
Many preparations have been utilized in the past for the production of hexagonal and amorphous BN. Most of these methods have involved the use of borates, boric acid, and cheap nitrogen-containing compounds, such as urea and ammonia. These methods have included, for example, high temperature heating of BCl.sub.3 and NH.sub.3 or urea in the vapor phase. The temperatures generally must be high enough to cause the simultaneous breakdown of by-product ammonium chloride to nitrogen, hydrogen, and hydrogen chloride.
One method has been to react boron trichloride with molten aluminum in the presence of nitrogen gas.
Yet another method for the production of BN has been the reduction of alkali cyanides.
High purity BN can also be made by the chemical vapor deposition (CVD) of BCl.sub.3 or B.sub.2 H.sub.6 and NH.sub.3 or by CVD of borazine and its derivatives at 900.degree. to 1500.degree. Centigrade. See Meller, supra, and Singh, Proc. Electrochem. Soc., pages 87-88, vol. 543 (1987).
U.S. Pat. No. 3,241,919, issued Mar. 22, 1966 to O'Conner, teaches the preparation of BN material showing a complete lack of three-dimensional order among the lamellae. These BN materials are described therein as "turbostratic" in nature. The method utilizes cyrstalline urea, orthoboric acid, and nitrogen gas, and eventually reaches a temperature of 800.degree. to 2000.degree. Centigrade.
U.S. Pat. No. 4,707,556, issued Nov. 17, 1987 to Paciarek et al., teaches the preparation of polymeric alkyl boron nitrides by a process comprising reacting chloroborazines with a disilazane.
Bradford et al., Inorg. Chem., 1, 99 (1962), has shown the reaction of N-lithioborazines or organo-haloborazines to prepare BN polymeric material.
However, the processes previously used to make or purify BN have required high temperatures of 900.degree. to 2800.degree. Centigrade or higher and tedious processing steps to achieve a BN product of high purity. Furthermore, often in chemically removing foreign matter such as by-products or unreacted starting materials, the BN undergoes reversion to H.sub.3 BO.sub.3.
There is thus a need for a low temperature method for the preparation of high purity BN without tedious processing steps.